i had a quick look for what could be causing it cos i didnt have a clue about it, looks like it could be caused by the pre-eclampsia
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http://www.babycenter.com/refcap/pregna ... 460.html#2 )
What could be causing this problem?
Experts don't always know what causes low amniotic fluid. It's most common late in your third trimester, particularly if you're overdue. The later in pregnancy the condition develops, the better the outlook for your baby. Here are some of the most common causes:
Leaky or ruptured membranes
A small tear in your amniotic membranes allows some fluid to leak out. This can happen at any point in your pregnancy but it's more common as you approach delivery. You may notice the leaking fluid yourself if you find that your underwear is wet, or your practitioner may discover it during an exam.
A ruptured membrane can increase both your and your baby's risk of infection because it provides a way for bacteria to enter the uterus. Occasionally, a tear will heal on its own, the leaking will stop, and the fluid will return to normal. (This is usually the case if the leaking happens after an amniocentesis.)
Placenta problems
A problem with your placenta, such as a partial abruption, keeps it from supplying enough blood and nutrients to your baby and causes him to stop recycling fluid.
Certain medical conditions
As mentioned above, certain conditions, such as chronic high blood pressure, preeclampsia, diabetes, and lupus, can result in low amniotic fluid levels.
Carrying twins or multiples
You're also at risk for low fluid levels if you're carrying twins or multiples. Oligohydramnios is likely in the case of twin-to-twin transfusion syndrome in which the donor twin suffers from too little amniotic fluid while the recipient twin creates too much.
Fetal abnormalities
If you're found to have low amniotic fluid in your first or second trimester, it may mean that your baby has a birth defect. If his kidneys aren't there or aren't developing properly (Potter's syndrome) or his urinary tract is blocked, your baby won't produce enough urine to maintain the level of amniotic fluid. A congenital heart defect can also cause this problem.